Unplug the refrigerator, the freezer
needs to be cold as normal is to test. Remove the rear inside freezer wall. There are coils behind that wall. Clipped to the bottom of that coil is a heater and it runs along the left side of that coil as well. Trace that heater to the wire connectors and separate them. Set your meter to any ohm setting then put one lead to each connection going to the heater. If the meter changes when you touch the heater connections then that heater is good. Changing is all that is important, a change is like when you touch the meter leads together. The meter changed. No certain reading just a change.
Clipped to the top of those coils is the defrost thermostat. This part must be cold to test. If its not cold make all the wiring safe and turn on the refrigerator until it is. Same test, a change is a good thermostat and no change is a bad thermostat.
If both these parts test good then the defrost timer is bad. The defrost timer is located in the panel inside the fresh food section where the temperature adjustment is. Take the screws out of that panel, remember to have the refrigerator unplugged.
Inside that panel you will find 1 part with a 4 wire connector on it. That will be the only part with a 4 wire connector on it. That is the defrost timer. If the parts in the freezer test good then the remaining and last part has to be bad. Replace the defrost timer.
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